The use of home water treatment systems to treat tap water continues to grow dramatically in the U.S. and abroad, in part because of heightened public awareness of the health concerns associated with the consumption of untreated tap water. Of particular concern are pathogens, which are microbes that cause disease. They include a few types of bacteria, viruses, protozoa, and other organisms. Some pathogens are often found in water, frequently as a result of fecal matter from sewage discharges, leaking septic tanks, and runoff from animal feedlots into bodies of water from which drinking water is taken. Bio-terrorism also poses a significant threat to water supplies.
Total Coliforms are a group of closely related bacteria that live in soil and water as well as the gut of animals. The extent to which total coliforms are present in the source water can indicate the general quality of that water and the likelihood that the water is fecally contaminated. Specific types of coliforms (i.e., fecal coliforms or E. coli) can present serious health risks. The Environmental Protection Agency (EPA) has set forth minimum standards for acceptance of a device proposed for use as a microbiological water purifier. Devices that claim removal of coliforms, represented by the bacteria E. coli and Klebsiella Terregina, must show a minimum 6-log reduction, 99.9999% of organisms removed, from an influent concentration of 1×107/100 ml.
Cryptosporidium is a single-celled microbe contained in a group generally known as protozoa. Cryptosporidium may cause a disease, cryptosporidiosis, when ingested. Cryptosporidiosis symptoms can range from mild stomach upset to life threatening disease in those who are immunocompromised (e.g., people with severely compromised immune systems). Oocysts are a stage in the life-cycle of some Cryptosporidium. In this stage, the Cryptosporidium can infect humans and other animals. The EPA requires removal of at least 99% of Cryptosporidium from water for qualified devices.
Giardia lamblia (commonly referred to as Giardia) are single-celled microbes contained in a group known as protozoa. When ingested, they can cause a gastrointestinal disease called giardiasis. Giardiasis is a frequent cause of diarrhea. Symptoms may include diarrhea, fatigue, and cramps. Waterborne giardiasis may occur as a result of disinfection problems or inadequate filtration procedures. Cysts are a stage in the life-cycle of some Giardia. In this stage, the Giardia can infect humans and other animals. Devices that claim cyst removal must show a minimum 3 log reduction, 99.9% of cysts removed, from an influent concentration of 1×107/L.
Viruses, including hepatitis A virus, rotaviruses, and Norwalk and other caliciviruses, are microbes that can cause serious illness. The EPA requires water purifiers to ensure a 4 log reduction, 99.99% of viruses removed, from an influent concentration of 1×107/L.
Two types of systems exist for the filtration of tap water. One type is pressurized, such as a faucet-mount system, and typically uses a porous carbon block as part of the filtration system. The other type is a low pressure system, such as a pitcher filter system, and typically uses activated carbon granules as part of the filtration system. However, few filtration materials are able to meet EPA standards for more than a few liters of water with filters of a reasonable size.
We have surprisingly found that a synergistic effect occurs when inorganic materials with high isoelectric points, such as magnesium salts, and activated carbon are bound by a low melt index binder. The resulting filter medium is very effective at removing microorganisms from large quantities of water, in filters small enough for point-of-use systems.
Magnesium salts have been used to remove polar materials from non-polar liquids by filtration. For example, U.S. Pat. No. 6,338,830 to Moskovitz and Kepner describes the use of Group IIA, Group IIIA, Group IVA, Group VA and transition metal oxide to remove contaminants from non-aqueous liquid or gas streams.
U.S. Patent Application No. 2002/0050474 to Munson and Roberts describes the use of magnesium silicate to remove polar impurities from used cooking oils.
Magnesium ions have also been used to promote cell survival. For example, U.S. Pat. No. 6,096,216 to Shanbrom describes the use of magnesium salts to preserve biological fluids during filtration through iodinated resin. U.S. Patent Application No. 2002/0053537 to Lucido and Shaffer describes the use of magnesium as a nutrient to feed microorganisms in a bioreacter.
International Patent Application WO 01/07090 to Hou et al. describes cationic polymers attached to substrates, including carbon blocks, for removing microorganisms.
Some prior art filters use biocidal resins and peroxides to kill microorganisms. For example, U.S. Pat. No. 4,361,486 to Hou and Webster describes the use of magnesium peroxide to oxidize soluble iron and inactivate microorganisms. A drawback to such filters is that the biocidal agent as well as the dead microorganisms pass through the filter and into the drinking water.
International Patent Application WO 02/076577 to Hughes broadly describes the use of magnesium compounds in carbon block form to remove microorganisms from a fluid, and is herein incorporated by reference. The purification material disclosed in Application WO 02/076577 removes microorganisms from fluids through adsorption to the magnesium compound. However, because the magnesium containing material only represents a small percentage of the surface area exposed to the fluid, the sites to which microorganisms can become adsorbed are few. Thus, the efficiency of the filter is limited, in that many microorganisms are not captured but merely pass through the filter. In addition, the adsorption sites quickly fill up, making adsorption difficult if not impossible and/or resulting in clogging of the filter pores ultimately resulting in a short filter life. For example, Application WO 02/076577 only discloses the ability to remove microorganisms from 500 ml of water. Moreover, the filter disclosed in Application WO 02/076577 is very large, with an outer diameter of 2.5 inches, an inner diameter of 1.25 inches, and length of 9.8 inches, making it unsuitable for many point-of-use purposes and in portable devices.
U.S. Pat. Nos. 4,753,728 and 5,017,318 to Vanderbilt et al. describe a filter constructed of powdered activated carbon bound by an ultra high molecular weight polyethylene binder, but which is only capable of capturing insignificant quantities of microorganisms.
U.S. Patent Application No. US 2003/0038084 to Mitchell et al. describes a filter composed of carbon particles heated in an oven in an atmosphere of ammonia that purportedly removes microorganisms through a combination of capturing fimbriae and surface polymers of the microorganisms in pores on the surface of the particular carbon particle, by adsorption and size exclusion.
What is needed is a more efficient filter medium capable of removing microorganisms to EPA standards from substantially larger quantities of water per unit filter medium than was heretofore possible.